Recently, surface-mounted components are required to be smaller and lighter in accordance with the recent multi-functioned communication devices.
One of such surface-mounted components is a duplexer for separating signals into the signals in the transmitting frequency band and the signals in the receiving frequency band, and dielectrics have been employed for conventional duplexers. However, since dielectric duplexers cannot be miniaturized in principle in the frequency range in the present communication standards, and the attenuation characteristic in the vicinity of the passband cannot be made steep, it has been impossible to achieve sufficient characteristics in the communication standards where the transmitting frequency band and the receiving frequency band are close in the vicinity.
Therefore, in recent years, there have been an attempt to utilize a filter including an surface acoustic wave element for a duplexer.
While surface acoustic wave elements have been used as interstage filters, they have low power handling capability to be used as duplexers.
However, since this problem of low power handling capability has been solved by ideas for modifying the structure and material of IDT electrodes, there have been surface acoustic wave duplexers coming up, (hereinafter abbreviated as “SAW-DPX”) having sizes smaller than dielectric duplexers and better attenuation characteristics in the vicinity of the passband than those of dielectric duplexers.
In general, a surface acoustic wave element is constructed by forming a plurality of exiting electrodes including an IDT electrode on a piezoelectric substrate. The period of the electrode fingers of the IDT electrode is almost determined by the sound speed of the material for the piezoelectric substrate and the frequency band in which the surface acoustic wave element is used. For example, when a surface acoustic wave filter used in a frequency band of 800 MHz is formed using single crystalline lithium tantalite as the piezoelectric substrate, the electrode finger period is about 4 um, that is, each of the width of one electrode finger and the distance (gap) between adjacent electrode fingers is about 1 um.
In addition, aluminum-based materials have been conventionally used for its ease of handling in manufacture and high conductivity.
In comparison to the surface acoustic wave filter including an IDT electrode comprising a large number of narrow electrode fingers arranged with narrow gaps in between, in a SAW-DPX, since a high frequency signal with an electric power of as large as 1 W is applied, the surface of the piezoelectric substrate vibrates physically strongly in response to the alternating electric field.
For this reason, since a great deal of stress is applied at a high frequency to the electrode, atoms of the material constituting the electrode migrate, whereby hillocks and cracks are generated in the electrode, finally resulting in destruction of the electrode. In particular, in the case of a material composed mainly of aluminum that constitutes the electrode, because of the presence of dangling bonds in grain boundaries of aluminum, only small quantity of energy is required for the movement of atoms, which causes greater migration.
Various approaches have been proposed on the electrode structure that can secure power handling capability in a surface acoustic wave element.
The first approach is adding metal elements in trace amounts to aluminum of the electrode material. The elements added to aluminum for the purpose of improving the power handling capability show two types of behaviors depending on the kind. The first behavior is to precipitate in the crystal grain boundaries of aluminum or to form an intermetallic compound with aluminum in the crystal grain boundaries of aluminum. Since these elements have a function to fill the dangling bonds of aluminum in the crystal grain boundaries, they have an effect to suppress migration of aluminum in the crystal grain boundaries. Elements having such functions include, namely, germanium, copper, palladium, silicon, and lithium.
The second behavior is to form solid solution with aluminum itself. Such elements work to prevent movement of dislocation that occurs in the crystal structure of aluminum caused by stress. Such elements include scandium, gallium, hafnium, zinc, zirconium, titanium, magnesium and the like. By adding at least one kind of these two kinds of elements to aluminum, the power handling capability of the surface acoustic wave element can be improved (for example, see Japanese Unexamined Patent Publication No. 1-80113, Japanese Unexamined Utility Model Publication No. 2-28120 and Japanese Unexamined Patent Publication No. 5-267979).
However, when the amounts of those elements added to aluminum are excessive, the resistance of the electrode becomes excessive, thereby applying electric power to the electrode results in a large heat generation. As a result, the thermal energy accelerates migration. Therefore, there should be proper amounts to be added.
The second approach is to eliminate the crystal grain boundaries themselves by single crystallization of aluminum (for example, see Japanese Unexamined Patent Publication No. 5-199062). Alternatively, if single crystallization is impossible, power handling capability can be enhanced by improving the degree of orientation of higher filling factor of aluminum crystals which hardly migrate. To achieve these objects, it is effective to provide an under layer film between the electrode made of aluminum and the piezoelectric substrate. For this under layer film, an intermetallic compound with aluminum, or a material whose heat of formation of the intermediate phase is positive may be chosen (for example, see Japanese Unexamined Patent Publication No. 4-090268).
The third approach is to relieve propagation of large stress in the electrode. Various techniques have been proposed for this purpose. One of them is minimizing the crystal grain size so as to disperse stress, because the larger the crystal grain size of aluminum is, the larger is the stress applied. Generally, it is believed that the size of a crystal grain is almost as large as the film thickness. Therefore, in order to obtain small crystal grains in an electrode with a predetermined thickness, a material that is different from aluminum (e.g. metal, nitride, silicide, etc.) may be inserted somewhere in the thickness direction (for example, see Japanese Unexamined Patent Publication No. 4-288718).
Another approach is proposed to insert a thick titanium layer between the electrode made of aluminum and the piezoelectric substrate (for example, see Japanese Unexamined Patent Publication No. 2002-368568). The thick titanium layer in this case does not have the function to orient aluminum as described referring to the second approach. This is because forming a thick film causes large irregularities on the surface. However, since titanium is a material with higher power handling capability than that of aluminum, inserting such a material between aluminum and a piezoelectric substrate can suppress propagation of stress to aluminum, whereby the power handling capability is improved.
In addition, the fourth approach is to use metal other than aluminum such as tantalum, gold, copper or the like as an electrode (for example, see Japanese Unexamined Patent Publication No. 9-98043).